Integrated Devices on a Common Compound Semiconductor III-V Wafer

ABSTRACT

A method of fabricating an integrated circuit on a compound semiconductor III-V wafer including at least two different types of active devices by providing a substrate; growing a first epitaxial structure on the substrate; growing a second epitaxial structure on the first epitaxial structure; and processing the epitaxial structures to form different types of active devices, such as HBTs and FETs.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional for Divisional Ser. No. 12/778,307, filed on May 12, 2010, which in turn is a divisional of Ser. No. 11/494,969, filed on Jul. 28, 2006. The contents of the earlier applications are incorporated by reference herein. This application also is related to the U.S. patent application Ser. No. 11/495,811, entitled “PROCESS FOR MANUFACTURING EPITAXIAL WAFERS FOR INTEGRATED DEVICES ON A COMMON COMPOUND SEMICONDUCTOR III-V WAFER”.

BACKGROUND

The integration of heterojunction bipolar transistors (HBTs) or bipolar junction transistors (BJTs) and field effect transistors (FETs) on a single silicon substrate is known in the art of BiCMOS. The integration of HBTs and FETs on a single chip has significantly reduced the size and cost of many electronic devices in use today. Silicon-based HBTs/BJTs and FETs, however, are known to have certain performance limitations that make them unattractive for some high-frequency analog applications, such as high efficiency cell phone amplifiers and ICs as employed in current wireless communication applications.

The fabrication of HBTs on GaAs and other III-V compound semiconductor substrates is also known and is attractive for both its performance, reliability and suitability for wireless applications. Integration of HBTs and FETs would enable higher levels of integration and would provide improvements in overall integrated circuit performance. However, few practical ways of integrating both HBTs and FETs onto a single GaAs substrate are known in the art.

One previously described method involved the growth of both HBT and FET structures on a substrate by selective MBE growth. However, this approach provided inconsistent results because of epitaxial (epi) growth interruption and epi re-growth.

Another previously described method provided a combination of HBTs and FETs on a substrate by using the emitter cap layer of an HBT as a FET channel. However, the method caused an unacceptably high emitter resistance of the HBT and parasitic effects associated with the base layer that degraded FET performance.

Other efforts have included the growth of an AlGaAs/GaAs HBT on top of a High Electron Mobility Transistor (HEMT) in a single growth process. This process merged a FET into the collector of the HBT through a single epitaxial growth with only limited success because of poor performance characteristics.

A number of other attempts have been made to integrate InGaP/GaAs HBTs with MESFET and HEMT structures. In these attempts, an InGaP layer was used as the channel for the FET devices. However, the channel had low mobility and saturation velocity with high linear resistance and poor high frequency performance.

Accordingly, a need exists for a method of manufacture that integrates HBT and FET devices on a single compound substrate selected from Group III-V materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table indicating the epitaxial layer sequence for an HBT-MESFET structure.

FIG. 2 depicts a substrate used for fabricating an integrated III-V semiconductor device used in an illustrated embodiment of the invention.

FIG. 3 depicts of substrate of FIG. 2 with a first epitaxial structure fabricated onto an upper surface.

FIG. 4 depicts the substrate of FIG. 3 with second and third epitaxial structures fabricated onto the upper surface.

FIG. 5 depicts a simplified view of the integrated III-V semiconductor device in accordance with an illustrated embodiment of the invention.

FIG. 6 depicts a wafer having a number integrated III-V semiconductor devices fabricated thereon in accordance with an illustrated embodiment of the invention.

FIG. 7 is a table indicating the epitaxial layer sequence for an HBT-PHEMT structure.

FIG. 8 is a flow chart of a fabrication process that includes in-situ substrate cleaning in accordance with an embodiment of the invention.

FIG. 9A is a graph showing the doping depth profile of a PHEMT-type epitaxial structure for which the in-situ substrate cleaning process was not performed.

FIG. 9B is a graph showing the doping depth profile of a PHEMT-type epitaxial structure for which an in-situ substrate cleaning process was performed.

FIG. 10 is a flow chart of a wafer processing cycle that includes a chamber pre-conditioning process according to an embodiment of the invention.

FIG. 11 is a graph comparing the sub-threshold leakage for PHEMT-type devices fabricated on epitaxial structures with and without the chamber pre-conditioning process.

SUMMARY

The present application relates to integrated devices on a common compound III-V semiconductor wafer.

As used herein, an epitaxial wafer includes a sequence of epitaxially grown layers on a semiconductor substrate. The various layers typically are lattice-matched and are selected for their particular physical and electrical characteristics. In some cases, the epitaxial wafer also may include one or more metamorphic or other layers.

In one aspect, the invention includes a method providing a substrate, growing a first epitaxial structure on the substrate, growing a second epitaxial structure on the first epitaxial structure, and growing a third epitaxial structure on top of the second epitaxial structure. The method further includes processing the epitaxial structures to form different types of active devices.

In another aspect, the invention includes a method of fabricating an integrated III-V semiconductor structure that includes at least two different types of active devices. The method includes providing a substrate and performing an in-situ substrate cleaning process to reduce the presence of contaminants on the substrate. A first epitaxial structure is grown on the substrate, and a second epitaxial structure is grown on the first epitaxial structure. The method includes processing the epitaxial structures to form different types of active devices.

According to a further aspect, a method of fabricating an integrated III-V compound semiconductor structure including at least two different types of active devices includes providing a first III-V compound semiconductor substrate in a reactor. At least two epitaxial structures for different types of devices are grown on the first substrate while the first substrate is in the reactor, and the first substrate is removed from the reactor. The method includes coating inner surfaces of the reactor to reduce the release of contaminants from those surfaces during subsequent process steps, and subsequently providing a second III-V compound semiconductor substrate in the reactor. At least two epitaxial structures for different types of devices are grown on the second substrate while the second substrate is in the reactor. The epitaxial structures on the second substrate are processed to form different types of active devices.

In yet another aspect, a semiconductor structure includes a substrate, a first epitaxial structure disposed on top of the substrate, and a second epitaxial structure disposed on top of the first epitaxial structure. An interface between the substrate and the first epitaxial structure is substantially free of contaminants. The epitaxial structures form portions of different types of active devices.

Some implementations may include one or more of the following features and advantages.

For example, the epitaxial process may enable growth of FET and HBT devices as combined structures on a single substrate.

The process may enable production (high volume, successive epitaxial growth runs) of FET/HBT structures that accommodate the conflicting requirements of each specific device type and device epitaxial parameters or constraints.

The process may enable incorporation of selective etch stops, either wet or dry, to selectively contact key layers within each device type.

The process may incorporate sacrificial “lift-off” layers that can be fully removed to eradicate one or either device type during subsequent device processing.

The process may, or may not, be a single continuous epitaxial deposition process.

The process may realize sharp, well defined interfaces and exhibit sufficient layer thickness control so that the FET may be implemented as a Pseudomorphic High Electron Mobility Transistor (PHEMT) device using epitaxially strained layers and/or delta doping techniques.

The process need not require any specific functionally shared layers between the HBT and the PHEMT.

The process may be employed for both planar and non-planar substrates to enable subsequent beneficial processing morphology.

In some implementations, the process is sufficient to achieve a highly resistive buffer for sufficient device isolation (DC and RF) within the environment generated by HBT epitaxial deposition. The process may include ex-situ or in-situ substrate preparation, control over the resistivity of buffer layers and buffer layer sequences.

The process may achieve highly doped contact layers for HBT emitters and HBT bases.

The process may be preferentially implemented in a rotating disk reactor (RDR) MOCVD process but also may be used for alternate MOCVD or MBE techniques.

The process may be used with GaAs, InP and related Group III-V compounds.

The process may encompass any specific HBT/BJT implementation (e.g., InGaP, AlGaAs emitter, GaAs, InGaAs, InGaAsN base).

Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The process for fabricating an integrated III-V semiconductor device is usually divided into two distinct technological stages or processes often performed by two different entities in the semiconductor industry. The first stage to produce a semiconductor wafer having a sequence of epitaxial layers with the desired physical and electrical properties. Such a process is described in U.S. patent application Ser. No. 11/495,811, entitled “PROCESS FOR MANUFACTURING EPITAXIAL WAFERS FOR INTEGRATED DEVICES ON A COMMON COMPOUND SEMICONDUCTOR III-V WAFER”.

The second stage is to process such an epitaxial wafer by lithographic techniques so that the desired device and circuit topology (including interconnections between devices are defined, followed by provisioning or deposition of passive components and electrical contacts, dicing the wafer into discrete integrated circuit chips (ICs), and packaging such ICs into an encapsulated package, a lead frame, or other type of package so that they may be mounted on a printed circuit board.

The present invention is directed to the second such technological process—the fabrication of two different types of active devices integrated on a common compound semiconductor III-V wafer having a suitable epitaxial structure, and the integrated semiconductor devices so formed by such a process. In particular, the present invention incorporates the steps of the first process of fabricating a semiconductor wafer having a sequence of epitaxial layers with the desired physical and electrical properties, followed by processing the epitaxial structures to form (i) different types of active devices; and (ii) integrated circuits formed by such interconnections of such active devices, optionally including passive components as well; all on the common compound semiconductor group III-V wafer.

In a specific example, a heterojunction bipolar transistor (HBT) and field effect transistor (FET) may be fabricated on the same substrate. In one embodiment, the HBT is fabricated first. Once the HBT has been fabricated, a passivation layer can be formed or deposited by standard semiconductor fabrication techniques over the HBT to protect the HBT. Once the passivation layer has been deposited over the HBT, the FET may be fabricated. In an alternative embodiment (to be described next), the process is reversed.

Details of a particular example for an integrated HBT-MESFET structure on a single substrate are provided in the table of FIG. 1. According to that example, various epitaxial layers are grown sequentially on a semiconductor substrate 10 (FIG. 2). The columns in the table of FIG. 1 indicate, respectively, the function of each layer, as well as the nominal thickness in angstroms (Å), the mole fraction and the carrier concentration (per cm³) for each layer in the particular implementation. In other implementations, those values may vary. In addition, other substrates and layers may be provided, and different types of devices may be formed in the resulting structure.

The substrate 10 can be based upon any Group III-V material (e.g., preferably a semi-insulating GaAs substrate). The set of epitaxial layers can be grown on the substrate using any known technique (e.g., VPE, MOCVD or MBE).

In one aspect of the invention, a low leakage buffer layer can be provided as a first step in creating a first epitaxial structure. The buffer layer may include, for example, one or more layers of undoped GaAs or AlGaAs layers. The particular buffer layer in the example of FIG. 1 has two sub-layers: an undoped GaAs layer on an undoped AlGaAs layer. The buffer layer can help initiate crystal growth and can provide the required pinch-off features of the FET. The buffer layer also can help trap unwanted and residual, electronically active impurities and provide electrical isolation for the subsequently formed devices. The buffer layer is particularly important for enabling the proper operation of FETs that are integrated on a common substrate with HBTs.

Next, a first epitaxial structure 12 for the FET may be grown (FIG. 3) over the substrate. The FET structure 12 may include, for example, a 50 nanometer (nm) thick undoped GaAs spacer layer and a doped GaAs channel layer of about 150 nm thickness. The channel layer may be doped, for example, with an n-type dopant having a doping concentration of about 2.0×10¹⁷ cm⁻³. The first epitaxial structure 12 also may include additional layers, such as an InGaP stop-etch layer and a GaAs contact layer for the FET.

A separation layer or stop-etch layer may be disposed over the FET layers as part of a second epitaxial structure 14. Alternatively, or in addition, a contact layer may be disposed over the first epitaxial structure 12 as part of the second epitaxial structure 14 (FIG. 4). The contact layer serves as the subcollector for the HBT device as well as a cap layer for the FET.

Although the particular example discussed here includes a layer that is common to the both the FET(s) and HBT(s), in other implementations the different types of devices can be formed in different layers such that they do not share a layer in common. Forming the different types of devices in different layers allows the electrical or other characteristics of the devices to be tuned independently and can provide greater flexibility in their design.

Next, in the particular example of FIG. 1, the other HBT layers can be grown over the contact layer as a third epitaxial structure 16. First, lightly doped n-type GaAs collector layers of about 1,000 nm may be grown, followed by a p-type GaAs base layer of about 100 nm. The GaAs layer may be doped, for example, with carbon to a level of about 4.0×10¹⁹ cm⁻³. An InGaP emitter layer of about 50 nm doped with silicon to a level of about 3.0×10¹⁷ cm⁻³ can be grown over the base layer.

The FET and HBT devices, including electrical contacts, can be fabricated in the foregoing structure using standard photolithography steps. FIG. 5 depicts a cross-sectional view of an integrated HBT/FET device. As shown, the elevated structure 32 a right side depicts a HBT, and the lower structure 34 depicted on the left side depicts a FET.

In the illustrated example of FIG. 5, contacts 18, 20, 22 for the sub-collector, emitter and base of the HBT can be formed using photoresist and etch steps.

A passivation layer can be formed or deposited by standard semiconductor fabrication techniques over the HBT device to protect the HBT device. A vertical barrier 30 also may be provided in the substrate through etch isolation or ion implantation techniques to isolate adjacent the FET and HBT devices. For example, the barrier can be created using a He+ ion implantation process.

Next, source and drain contacts 24, 26 of the FET are formed on the wafer using, for example, a metal evaporation and lift-off process. A channel recess etch may be performed to expose the FET layer. A gate 28 then can be formed in the exposed FET layer using another appropriate technique (e.g., e-beam evaporation).

FIG. 6 depicts a wafer 50 containing a number of fabricated devices. Once diced, the wafer 50 provides a number of chips 52 each of which contains, for example, an integrated HBT and FET. Each chip 52 may include multiple HBTs and FETs as well as one or more passive devices that, together with the active devices, are interconnected to form an integrated circuit. The chip 52 can be encapsulated in a package to provide external electrical connections. The package may be, for example, a plastic RF module. Other types of packages can be used as well.

Structures with multiple epitaxially grown structures also can be processed to form other types of integrated active devices. For example, a first epitaxial structure can provide the layers for a PHEMT and a second epitaxial structure can provide the layers for a HBT.

Details of a particular example for an integrated HBT-PHEMT structure on a single substrate are provided in the table of FIG. 7.

A wafer with the foregoing structure can be processed to form the active HBTs and PHEMTs as well as any passive devices. The wafer then is diced to form individual chips each of which contains an integrated circuit that includes an HBT and a PHEMT. The chip can be encapsulated in a package with leads to provide external electrical connections.

In some implementations, each chip includes multiple devices in the various epitaxial structures. For example, each chip may include multiple PHEMT devices formed in the first epitaxial structure.

Device characteristics and performance can be adversely impacted, for example, by the presence of surface contaminants at the interface between the substrate 10 and the first epitaxial structure 12. Examples of contaminants include silicon (e.g., resulting from polishing the GaAs substrate), tellurium (e.g., from previous wafer runs in which tellurium-doped InGaAs epi-layers are grown), or excessive levels of O₂. In particular, such contaminants can adversely impact the performance of FETs such as PHEMTs. Also, excessively aggressive etching techniques (e.g., temperatures or flow rates that are too high) can cause mass transport from exposed reactor surface deposits to the substrate. The consequence of such mass transport is formation of conductive paths at the substrate-epi layer interface or increased substrate surface roughness or both, resulting in poor device performance (e.g., sub-threshold leakage) and poor isolation between devices on the same chip.

According to an aspect of the invention, in order to reduce the amount of contaminants that may be present at the substrate-epi interface, an in-situ substrate cleaning process can be performed. The cleaning process, an example of which is described in greater detail below, can be used to process wafers in a MOCVD reactor in which a platter is used to hold the wafers being processed. The cleaning process also can be used in other wafer processing equipment.

Preferably, the in-situ cleaning process is performed before growing epitaxial layers on the substrate 10 and can facilitate back-to-back wafer processing runs. The cleaning process can help remove the native contamination from the substrate while not inducing mass transport from the reactor deposits to the substrate. Thus, the cleaning process can result in a clean substrate-epilayer interface that provides good device isolation.

According to a particular implementation illustrated in FIG. 8, a wafer is loaded into the reactor (block 100). The cleaning process exposes the GaAs substrate to a halide-based etchant (e.g., HCl, Cl₂, AsCl₃, PCL₂, AsBr₂), in combination with atomic hydrogen. In a particular implementation, a chlorine-based etchant such as AsCl₃ is used. In that case, the substrate can be exposed, for example, to AsCl₃ and AsH₃ at an elevated temperature (e.g., in the range 400-800° C.). The cleaning process can include an AsH₃ exposure step (block 102), an AsCl₃ and AsH₃ exposure step for etching the substrate (block 104), followed by an AsH₃-only exposure for surface morphology recovery (block 106). In this example, the atomic hydrogen is obtained from the decomposed AsH₃. After performing the in-situ substrate cleaning process, the various epitaxial layers can be grown (block 108).

For particular implementations in which multiple wafers are processed in a high-speed, rotating disk reactor, a temperature of about 700° C. can be advantageous for the substrate cleaning process. At higher temperatures, tellurium may be released from layers in previous wafer processing runs. Suitable pressures are in the range of 10 Torr to atmospheric pressure, although a range of about 50-80 Torr is particularly well-suited for some applications. The exposure to the etchant should be long enough to clean the surface without excessively damaging the surface morphology. In some cases, an exposure time of about fifteen seconds to several minutes at a flow rate of about 100 cc³ can be used at appropriate pressure and temperature. The foregoing factors may vary from reactor to reactor and may vary depending on the configuration of the platter that holds the wafers. Thus, in general, the exposure time, flow rate, pressure and temperature for the cleaning process can be adjusted depending on the particular geometry, composition and thickness of the wafer to achieve desired device performance.

FIGS. 9A and 9B are graphs showing the doping profile of a PHEMT-type epitaxial structures. The horizontal axis indicates depth in angstroms, where the origin (“0”) corresponds to the top surface an AlGaAs barrier layer. The peak doping concentration (which in the illustrated example is somewhat greater than 1E+18 cm⁻³) corresponds to the InGaAs channel.

FIG. 9A shows the doping profile of a PHEMT-type epitaxial structure for which an in-situ halide-based substrate cleaning process was not performed. A peak in the doping profile appears at a depth of about 4,000 angstroms, which corresponds to the interface between the GaAs buffer layer and the GaAs substrate. Such a peak, which indicates the presence of charge resulting from contaminants at the buffer-substrate interface, is undesirable and can adversely impact FET device operation and isolation between devices. The in-situ substrate cleaning process can result in a dopant concentration at the interface of the substrate and the epitaxial structure that is between ten and hundred times less than it would be in the absence of the in-situ cleaning process.

FIG. 9B show the doping profile of a PHEMT-type epitaxial structure for which an in-situ chlorine-based substrate cleaning process was performed. As can be seen, the doping profile beyond a depth of about 1,500 angstroms continuously decreases with no additional peaks, indicating a significant reduction in the contaminants present at the buffer-substrate interface.

According to another aspect of the invention, in implementations using an MOCVD or other chamber in which semiconductor wafers are processed, it may be desirable to seal exposed surfaces of the reactor between wafer processing runs so as to reduce the release of contaminants from those surfaces during subsequent growth steps. Examples of reactor surfaces from which contaminants may be released include, but are not limited to, the reactor liner, the walls and ceiling of the reactor, the spindle, probes and screens, as well as a platter that holds the semiconductor wafers. The release of contaminants from such surfaces can be particularly problematic for FET-layers formed during a wafer processing run that follows a previous wafer processing run in which layers for HBT devices were formed.

As illustrated by FIG. 10, a first wafer processing run is performed (block 110). The wafer processing run may include, among other steps, loading semiconductor substrates (i.e., wafers) into the reactor, growing one or more epitaxial layers on the substrates and removing the substrates from the reactor. Before starting the next wafer processing run, the chamber pre-conditioning process is performed. In the illustrated implementation, the chamber pre-conditioning process includes placing one or more dummy GaAs (or other compound III-V semiconductor) wafers on the platter to cover the pockets in the platter (block 112), heating the platter to an elevated temperature (e.g., about 650° C.) (block 114), and epitaxially growing a thin coating of GaAs (or other compound III-V semiconductor) on the inner surfaces of the reactor (block 116). Such a thin coating (i.e., less than about 3 microns (μm)) can encapsulate any contaminants, such as tellurium, that may be present on the inner surfaces of the reactor. For some implementations, a coating having a thickness of about 0.25 μm is sufficient. Preferably, the sealing process should be performed as quickly as possible to reduce the overall manufacturing cycle time. In a particular implementation, the process can be completed in less than about five minutes. After growing the coating, the platter is cooled (block 118), and the dummy wafers are removed from the reactor (block 120). A new batch of III-V semiconductor substrates then can be processed in the reactor (block 122).

It is desirable to cover the pockets of the platter with dummy wafers (e.g., GaAs or silicon) during the chamber pre-conditioning process so as to increase the useful lifetime of the platter. However, the chamber pre-conditioning process can be performed without covering the pockets.

For fabrication processes in which both the in-situ substrate cleaning process and the chamber pre-conditioning process are performed, the substrate cleaning process can be performed more aggressively, for example by using higher temperatures (e.g., 700-800° C.). The higher temperatures can allow any in-situ thermal or chemical cleaning process to be performed more quickly without risk that contaminants on the platter will be transferred to the surface of the substrate. Performing such processes more quickly can result in shorter manufacturing cycles.

The foregoing substrate cleaning process and chamber pre-conditioning process can be particularly useful when fabricating an integrated III-V semiconductor structure with epitaxial structures that include at least two different types of active devices. Reducing the contaminants at the substrate-epitaxial layer interface, and possibly at other interfaces as well, can significantly improve device performance.

FIG. 11 illustrates voltage-versus-current curves for PHEMT-type devices in an epitaxial structure. The measurements were obtained using a mercury probe. The curve 120 corresponds to a device fabricated in an epitaxial structure which was grown without performing the in-situ substrate cleaning process and the chamber pre-conditioning process. As can be seen from the curves, the device exhibited significant sub-threshold current leakage. In contrast, the curve 122 corresponds to a device fabricated in an epitaxial structure which was grown after performing the chamber pre-conditioning process and the in-situ substrate cleaning process. The curve 122 indicates a significant reduction in sub-threshold current leakage.

Other implementations are within the scope of the claims. 

1-46. (canceled)
 47. A III-V compound semiconductor structure comprising epitaxial structures that include an integrated pair of different types of active devices, the semiconductor structure comprising: a semi-insulating substrate of a compound semiconductor III-V material; and a first compound semiconductor III-V epitaxial structure disposed on the substrate, the first epitaxial structure forming at least a portion of one of the active devices; wherein a concentration profile of a contaminant in the semiconductor structure across an interface between the substrate and the first epitaxial structure decreases substantially smoothly in a direction from the substrate to the first epitaxial structure, wherein the contaminant is silicon or oxygen.
 48. The III-V compound semiconductor structure of claim 47 wherein the concentration of the contaminant at the interface is less than 10¹⁶ cm⁻³.
 49. The III-V compound semiconductor structure of claim 47 wherein the epitaxial structures include an integrated pair of HBT and FET transistors.
 50. The III-V compound semiconductor structure of claim 49 wherein the HBT and FET transistors share a common compound semiconductor III-V epitaxial layer in one of the epitaxial structures.
 51. The III-V compound semiconductor structure of claim 47 further including: a second compound semiconductor III-V epitaxial structure over of the first epitaxial structure, and a third compound semiconductor III-V epitaxial structure disposed over the second epitaxial structure, the second epitaxial structure forming at least a portion of a second one of the active devices of a different type than the first active device; wherein the first and second active devices share a common compound semiconductor III-V epitaxial layer in one of the epitaxial structures.
 52. The III-V compound semiconductor structure of claim 51 wherein the concentration of the contaminant at the interface between the substrate and the first epitaxial structure is less than 10¹⁶ cm⁻³.
 53. A III-V compound semiconductor structure comprising epitaxial structures that include an integrated pair of different types of active devices, the semiconductor structure comprising: a semi-insulating substrate of a compound semiconductor III-V material; and a first compound semiconductor III-V epitaxial structure disposed on the substrate, the first epitaxial structure forming at least a portion of one of the active devices; wherein a concentration profile of a contaminant in the semiconductor structure across an interface between the substrate and the first epitaxial structure decreases substantially smoothly in a direction from the substrate to the first epitaxial structure, wherein the contaminant is a chemical element of the periodic table whose presence at or near the interface contributes electrical charge.
 54. The III-V compound semiconductor structure of claim 53 wherein the contaminant is silicon.
 55. The III-V compound semiconductor structure of claim 53 wherein the concentration of the contaminant at the interface is less than 10¹⁶ cm⁻³.
 56. The III-V compound semiconductor structure of claim 53 wherein the epitaxial structures include an integrated pair of HBT and FET transistors.
 57. The III-V compound semiconductor structure of claim 56 wherein the HBT and FET transistors share a common compound semiconductor III-V epitaxial layer in one of the epitaxial structures.
 58. The III-V compound semiconductor structure of claim 53 further including: a second compound semiconductor III-V epitaxial structure over of the first epitaxial structure, and a third compound semiconductor III-V epitaxial structure disposed over the second epitaxial structure, the second epitaxial structure forming at least a portion of a second one of the active devices of a different type than the first active device; wherein the first and second active devices share a common compound semiconductor III-V epitaxial layer in one of the epitaxial structures.
 59. The III-V compound semiconductor structure of claim 58 wherein the concentration of the contaminant at the interface between the substrate and the first epitaxial structure is less than 10¹⁶ cm⁻³.
 60. A III-V compound semiconductor structure comprising epitaxial structures that include an integrated pair of different types of active devices, the semiconductor structure comprising: a semi-insulating substrate of a compound semiconductor III-V material; and a first compound semiconductor III-V epitaxial structure disposed on the substrate, the first epitaxial structure forming at least a portion of one of the active devices; wherein a concentration profile of a dopant material in the semiconductor structure across an interface between the substrate and the first epitaxial structure decreases substantially smoothly in a direction from the substrate to the first epitaxial structure, and wherein the interface between the substrate and the first epitaxial structure is substantially free of a contaminant that was present, during formation of the first epitaxial structure, in a chamber in which the first epitaxial structure was formed.
 61. The III-V compound semiconductor structure of claim 60 wherein the concentration of the dopant material at the interface is less than 10¹⁶cm⁻³.
 62. The III-V compound semiconductor structure of claim 60 wherein the contaminant is tellurium.
 63. The III-V compound semiconductor structure of claim 60 wherein the dopant material is silicon.
 64. The III-V compound semiconductor structure of claim 60 wherein the epitaxial structures include an integrated pair of HBT and FET transistors.
 65. The III-V compound semiconductor structure of claim 64 wherein the HBT and FET transistors share a common compound semiconductor III-V epitaxial layer in one of the epitaxial structures.
 66. The III-V compound semiconductor structure of claim 60 further including: a second compound semiconductor III-V epitaxial structure over of the first epitaxial structure, and a third compound semiconductor III-V epitaxial structure disposed over the second epitaxial structure, the second epitaxial structure forming at least a portion of a second one of the active devices of a different type than the first active device; wherein the first and second active devices share a common compound semiconductor III-V epitaxial layer in one of the epitaxial structures. 